Forward osmosis (FO) is a technology currently being explored for desalination of seawater. Unlike reverse osmosis (RO) processes, which employ high pressures ranging from 400-1100 psi to drive fresh water through a membrane, forward osmosis uses the natural osmotic pressures of salt or polymer solutions, called ‘draw solutions’, to effect fresh water separation. A draw solution having a significantly higher osmotic pressure than the saline feed-water, flows along the permeate side of the FO membrane, and water naturally transports itself across the membrane by osmosis. Osmotic driving forces in FO can be significantly greater than hydraulic driving forces in RO, leading to higher water flux rates and recoveries. Thus, it is a low-pressure system, allowing design with lighter, compact, less expensive materials. These factors translate in considerable savings, both in capital and operational costs.
Joint research by Yale University and Oasys Inc, under an Office of Naval Research grant, compared forward osmosis to reverse osmosis processes, and found superior performance and flux rates. Based on these studies, Oasys developed a forward osmosis process using ammonium bicarbonate aqueous solutions as the draw down liquids. Other FO processes have been proposed, using either magnesium chloride draw solutions, polymeric draw solutions based on polyethylene glycols, volatile solutes like dimethyl amines, sulfur dioxide or aliphatic alcohols, or bivalent/precipitable salts like aluminum sulfate/calcium hydroxide (Modern Water, UK). Glucose or sucrose have been used as solutes for the draw solution, which can then be ingested after suitable dilution (Hydration Technologies International Inc). Polymeric draw solutions have also been developed based on polyethylene glycols (PEGs) and polypropylene glycols (PPGs).
Solutions of magnesium chloride, ammonium chloride, calcium chloride in water, and polymers like PEG/PPG solutions in water generate very high osmotic pressures, in the range of 300-400 atm, based on their concentration. The ionic salts mentioned above, as well as sodium and potassium bicarbonates, also do not decompose or scale at the temperatures contemplated herein, while the water in the salt solution can be substantially boiled off by the application of low-temperature waste heat, thus regenerating the concentrated salt solutions needed for hydro-osmotic power generation. The preferred draw solute for this application would be the ionic chlorides of magnesium or calcium, due to their very high osmotic potentials at a concentration of 2.5M to 3.0M, as well as the minimized scaling of these salts at steam temperatures. The use of these salts also enables the temperature in the boiler/heat exchanger to be higher, called the Top Brine Temperature (TBT) to around 125-150° C., which increases the efficiency of the boiler. However, the main drawback in the use of these concentrated ionic solutions is the need to boil off and recover the water of dilution, since the latent heat of vaporization of water is around 970 Btu/lb of water to be removed, a substantial energy penalty.
Similarly, polymeric draw solutions also generate very high osmotic potentials, and are also not volatile, with very high boiling points (≈230° C.), suitable for power generation from low-temperature waste heat. A polyethylene glycol (PEG) solution generates very high osmotic pressures for its solutions in water, depending on its concentration. Thus, a 95% solution in water of PEG 400 at 20° C. has a calculated osmotic pressure of 658 atm; for PEG 600, it is 977 atm; for PEG 2000, it is 2,540 atm.
Polyethylene glycols (PEGs), polymers of ethylene glycol (EG), have been used in industry to produce very high osmotic pressures, in the order of tens of atmospheres. In comparison, seawater (3.5% NaCl) has an osmotic pressure of only 28 atms at 25° C. PEGs are hypotonic by nature, and absorb water exceedingly well. The hydrogen bonding between water molecules and the electron-rich ether oxygen in the EO (ethylene oxide) monomer enables almost 2.5-3.0 molecules of water to be coordinated with each EO monomer, leading to high osmotic pressures. Thus, the greater the number of EO monomers in the PEG molecule, the greater the osmotic pressure exhibited.
While the PEGs used in these copolymers are linear in structure, and increase in melting point and viscosity as the chain-length increases, there are other forms of PEGs available, with different geometries, that are termed branched or multi-armed PEGs. Branched PEGs have 3-10 PEG chains emanating from a central core group. Star PEGS have 10 to 100 PEG chains emanating from a central core group, while comb PEGs have multiple PEG chains grafted onto a polymer backbone. Such branched PEGs allow more EO groups in the polymer, while remaining in the liquid state and having lower melting points and viscosity than comparable linear PEGs with the same number of EO monomers. Thus, the use of such PEG geometries can enable higher water absorption, while retaining the practicality of using higher number of EO monomers for water molecule interaction by hydrogen bonding, leading to high osmotic pressures. An additional property of these branched PEG polymers, as described in co-pending U.S. patent application Ser. No. 15/271,175, filed Sep. 20, 2016, and Ser. No. 15/272,406, filed Sep. 21, 2016, the entire contents of each of which are incorporated herein by reference, is also the ability to phase-separate from water by suitable amine-termination of the branched ends of these polymers and subsequent absorption of carbon dioxide.
Suitably engineered polymers enable high flux rates against fresh water across the FO modules, while efficiently phase-separating at temperatures associated with low-temperature heat (≈75-95° C.), without inordinately large heat transfer surfaces. Some such polymers would be block or random branched co-polymers of ethoxylate-propoxylates like sorbitol ethoxylate-propoxylates, sorbitan ethoxylate-propoxylates, glycerol ethoxylate-propoxylates, trimethylolpropane ethoxylate-propoxylates, pentaerithritol ethoxylate-propoxylates, glucose and sucrose ethoxylate-propoxylates, other poly-hydric polymers, and similar branched derivatives of these ethoxylate-propoxylate co-polymers.
Modification of these polymer derivatives by amine-termination enables them to undergo phase-separation from water, or inverse solubility in water, by absorption of CO2, as described in co-pending U.S. patent application Ser. No. 15/271,175, filed Sep. 20, 2016, and Ser. No. 15/272,406, filed Sep. 21, 2016, the entire contents of each of which are incorporated herein by reference. Use of such polymers, with their high osmotic pressures, and their property of inverse solubility with water by CO2 absorption, can be used for hydro-osmotic power generation.
A great quantity of energy can be potentially obtained when waters of different salinities are mixed together. The harnessing of this energy for conversion into hydro-osmotic power can be accomplished by means of a technology called Pressure Retarded Forward Osmosis (PRFO). This technique uses a semi-permeable membrane to separate a less concentrated solution, or solvent, (for example, fresh water) from a more concentrated and pressurized solution (for example an osmotic draw agent), allowing the water to pass to the concentrated solution side. The difference in osmotic potential between two solutions, separated by a semi-permeable membrane, yields a pressure differential, which is similar to the effect of gravity in creating potential energy (static head) for conversion to hydroelectric energy. Normal hydropower plants use the static head of water in dams to yield energy when the water is allowed to run through turbine generators. Similarly, osmotic pressure differentials can also be used to drive hydro-turbine generators to create energy. The additional fluid volume due to the permeation of water increases the pressure on the permeate side, which is depressurized in a hydro-turbine to produce power—thus the term ‘hydro-osmotic power’.